Report – CHEM120 – Stoichiometry of Chemical Reactions

Stefan Martensson C0347318

Procedure: Please refer to handout ‘Experiment #3’ and page 10-16, Chemistry 120 Lab Manual, 2009 Edition, Camosun College.

Theory: The Law of Constant Composition states that ‘the proportions of the elements in a compound are always the same, no matter how the compound is made’.

E.g. The reaction between heated copper and sulphur making copper sulphide (Cu2S).

Where we discovered by letting a copper thread be extensively heated with abundance of sulphur in a crucible until all sulphur had either fully reacted with the copper or burned off as sulphur dioxide.

The copper was then weighed and mass was recorded. More sulphur was added to the crucible and copper thread and was yet again set to heat up. This so the next measurement would indicate if a full reaction had occurred (about the same mass as the measurement after the first heating).

The mass after heating subtracted from the initial mass indicated by the law of constant proportion the amount of sulphur reacted with the copper. From this we can calculate the molar mass proportions of the copper and the sulphur in the copper sulphide and from that determine the empirical formula as shown below.

Data and Results:

Table 1 – Determining the Empirical Formula of Copper Sulphide
Mass of Crucible 32.8116g
Mass of Crucible + Copper 33.8008g
Mass of Copper 0.9892g
Mass of Crucible + Copper Sulphide 34.0571g
Mass of Copper Sulphide 1.2455g
Mass of Sulphur Reacted 0.2563g
Moles of Sulphur Reacted 0.007992mol
Mole Ratio of Cu:S 2:1

Table 2 – Determining the Empirical Formula of Copper Bromide
Mass of Flask 77.2820g
Mass of Flask + Copper Bromide A 78.3141g
Mass of Copper Bromide A 1.0321g
Mass of Flask + Copper Bromide B 77.9374g
Mass of Copper Bromide B 0.6554g
Moles of Copper Bromide B 0.004569mol
Mass of Bromide Drive Off 0.3767g
Moles of Bromine Driven Off 0.002357mol
Moles of Copper in Copper Bromide A 0.002024mol
Moles of Bromine in Copper Bromide A 0.004902mol
Mole Ratio of Cu:Br in Copper Bromide A 1:2


Reaction 1: Cu + S à[heat]à CuxSy

Mass of Copper Sulphur: 34.0571g-32.8116g=1.2455g

Moles of S reacted: 1.2455g-0.9892g=0.2563g

Moles of S reacted: 0.2563g/32.07g/mol=0.007992mol

Moles of Cu: 0.9892g/63.55g/mol=0.01557mol

Mole ratio Cu : S = 0.007992:0.01557 à = Empirical Formula Cu2S

Reaction 2: CuxBry (A) à Cu1Br1 (B)  + Br

Molarmass: Cu1Br1 (B): 63.55+79.90=143.45g/mol

Moles Cu1Br1 (B):0.6554g/143.45g/mol à Cu1Br1 (B)=0.004569mol

Therefore number of moles Cu in Cu1Br1 (B) = Number of moles Cu1Br1 (B)  * 1mol Cu/1mol Cu1Br1 (B) that in turn equals the number of moles of Cu in CuxBry (A).

Moles of Cu in Cu1Br1 (B): 0.004569mol * 63.55g/mol / 143.45g/mol = 0.002024mol

Therefore number of moles Br in Cu1Br1 (B) = Number of moles Cu1Br1 (B)  * 1mol Br/1mol Cu1Br1 (B).

Moles of Br in Cu1Br1 (B): 0.004569mol * 79.90g/mol / 143.45g/mol = 0.002545mol

Mass of Br driven off equals mass of CuxBry (A) – mass Cu1Br1 (B).


Moles of Br driven off equals mass of Br2 driven off * 1mol/159.80g/mol.

0.3767g / 159.80g/mol = 0.002357mol

Total moles Br in CuxBry (A) = number of moles Br in Cu1Br1 (B) + number of moles driven off.

0.002545mol + 0.002357mol = 0.004902mol

Empirical formula of CuxBry (A) = mol Cu: Total mass of Br.

0.002024mol Cu : 0.004902mol Br à Due to some decomposition a 1:2 relationship could be argued

= Empirical Formula ‘CuBr2


Part A – [a]

Adding more sulphur than recommended to the crucible would not affect the reaction. Reason being we wanted to have the copper react with as much sulphur as needed and the excess would naturally burn off as sulphur dioxide.


We would notice this after the second heating measurement. If there had not been a full reaction due to too strongly heated sulphur the recordings of mass would differ and thus the molar mass relationship between copper and sulphur would be a bit off, with copper having a higher proportion, but the total mass being lower than expected due to the sulphur being burnt off as sulphur dioxide instead of creating copper sulphide.


This error would be simple to correct by just adding more heat and time. However, if a full reaction would not occur the mass of the unfinished transformation of copper to copper sulphide would make the string a tad lighter in mass due to the incomplete reaction, similar to [b]. You would also have rests of sulphur in the crucible still left over. If you would take the incomplete copper sulphide string and the rest of the molten sulphur and weighed that, it would show a mass much higher than expected and do the opposite to [b] by indicating a higher proportion of sulphur than copper.

Part B – [a]

This would result in an incomplete reaction. The CuxBry (A) being a heavier molecule would shift the mass proportions to favour the bromine’s proportion since the bromine is heavier than copper.


I am not sure if it will cause a different reaction, perhaps an oxidation, but the measurement would be way off.


Empirical Formula part 1: Cu2S & Empirical Formula part 2: CuBr2

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